Adjustable frequency drives to control induction motors based on a pulse width modulator (PWM) voltage source inverter are well known. The PWM generates signals to control conduction of electronic switching devices in a power output driver circuit that supplies power to the motor. These switching devices could be power transistors, MOSFETs, IGBTs, GTOs, or other power devices that are connected across a DC bus in series connected pairs for each phase of the motor. These devices are complementary switched for each phase, meaning that only one of the pair of devices will be on at any instant in time. A common type of PWM utilizes a sine wave as a reference serving as the voltage command that is modulated by a triangular waveform of constant amplitude. Whenever the voltage command is larger than the triangular waveform, the appropriate switching device is turned on to apply a positive voltage from the DC bus to the output, and if the voltage command is less than the triangular waveform, the appropriate switching device is turned on to apply a negative voltage from the DC bus to the output. This type of PWM constitutes an open loop control of the output voltage and does not compensate or regulate the output voltage for nonlinearities caused by changes in the DC bus or characteristics of the switching devices.
These nonlinearities can be categorized into different causes that are the results of the techniques used to generate the voltage commands. A quantization effect occurs in digital systems since timing signals occur at discrete time intervals and so exact values for all pulse widths may not be attainable. To prevent a short circuit of the DC bus, a fixed lockout or deadtime is added to the time that one of the pair of devices is turned-off and the other device is turned-on. This deadtime is chosen to allow for finite turn-off and turn-on times of the complimentary devices and to insure that one of the pair is completely off before the other is turned on. This deadtime will result in a distortion in the voltage waveform since the output of the inverter at this time will not be controlled by the switching devices but will be a function of the output load current and the power factor of the motor. Related to deadtime is a minimum dwell time requirement for the switching devices to insure that they are completely off before they are commanded to turn on again. There may also be minimum on-times to ensure that devices are completely turned on and snubber circuits are discharged. When the devices are turned on there is an additional voltage error due to non-zero voltage drops across the devices. As a result there can be an error if the PWM control calculates a pulse width that is shorter than the dwell time.
The result of these nonlinearities will be an error voltage between the command voltage from the PWM and the actual output voltage that will have the appearance of an offset voltage.
To eliminate some of the effects of deadtime, various methods have been devised to provide some type of hardware or software compensation circuit. A common method is to insert a fixed or predetermined offset to the PWM signals according to the load current polarity to correct for the distortion caused by the deadtime. This type of closed loop voltage regulator will be effective for removing some of the nonlinearities of the drive system. However, since it functions outside of the PWM generator, it can not compensate for variations in the DC bus voltage coupled through the DO link or other causes of delays. Also, there will be errors in sampling of the output current. These errors result in a reduction in the fundamental voltage component and an increase in lower order harmonics which will cause excessive ripple current and torque pulsations in the motor. There will be a greater tendency towards instability for motors that are lightly load.
Another type of compensation scheme as outlined in U.S. Pat. No. 5,099,408 modifies the pulse widths of the PWM signals by adding or subtracting a predetermined amount to the pulse width on the basis of the polarity of the current as determined by detected zero crossings of the output current. The modification occurs in only one phase and also does not compensate for variations in the DC link. Its main objective is to correct for the influences of deadtime only.
The present invention eliminates these and other problems without loss of performance or reliability.